Cyanoacrylate is the generic name for a family of resistant fast acting adhesives based on esters of 2-cyanoacrylic acid. The structure of the monomer is as follows:
wherein R is generally an alkyl group such as, for example, methyl, ethyl, butyl or 2-octyl.
Such compounds are well known for some time, as disclosed in, for example, the manual S. Ebnesajjad Ed., Adhesives Technology Handbook, William Andrew, Norwich, 2008.
These compounds are ethylene monomers 1,1-disubstituted with electron withdrawing groups, so that they olefins are highly reactive towards nucleophiles for its electron deficiency. Their structure allows an extremely easy polymerisation thereof initiated by any nucleophilic or basic species which is inherently present in the medium or substrate to be bonded.
The industrial process for the preparation of cyanoacrylates is based on the well-known Knoevenagel reaction involving the reaction of a cyanoacetate with a source of formaldehyde, for example, paraformaldehyde or trioxane, with an appropriate cyanoacetate in basic medium. Reaction conditions inevitably result in a prepolymer, which must be subjected to a thermal depolymerization at high temperature, high vacuum and acidic conditions. The pure monomers are usually re-distilled and stabilized by trace quantities of acids, to avoid an undesired polymerization. Due to the strong conditions of the depolymerization process, this procedure is not appropriate for monomers that include thermally labile substituents.
In the prior art several improvements to such method have been described, as disclosed, for example, in the U.S. Pat. No. 6,245,933, and the references cited therein.
Despite the improvements introduced, the process based on the Knoevenagel reaction is efficient only for lower alkyl cyanoacrylates, for example, ethyl cyanoacrylate which constitutes nearly more than 90% of the production of cyanoacrylates to be used in fast curing adhesives. In these cases it is possible to implement waste recycling systems that allow obtaining satisfactory yields.
For other cyanoacrylates, the yields obtained by means of the Knoevenagel reaction are substantially low. For example, 2-phenylethyl cyanoacrylate is obtained with 39%, as disclosed in the Japanese patent application JP-A-06-192202; ethyl glycolate cyanoacrylate is obtained with a yield of 23%, as disclosed in the European patent application EP-A-0127855; trimethylsilylmethyl cyanoacrylate with a 30% yield as disclosed in the European patent application EP-A-0459617; tetrahydrofurfuryl cyanoacrylate with a 64% before being distilled, as disclosed in the U.S. Pat. No. 4,321,180; and alkoxyethyl cyanoacrylates with a yield ranging from 14% for hexoxy group, to 45% for the methoxy group, as disclosed in Muzrahi et al., Acta Biomater., 2011, 7(8), 3150-3157.
However, the applicative properties on an adhesive depend largely on the ability to combine in a single formulation different monomers to achieve certain effects, in the same way that different acrylic monomers are combined in a paint to achieve, for example, increased hardness and durability.
Additionally, it should be born in mind that other kind of monomers which polymerize instantaneously, methylidene malonates, which also belong to the class of 1,1-disubstituted ethylene monomers, are difficult to be efficiently obtained by a process based on the Knoevenagel reaction. Improvements of this process are disclosed in the international patent application WO-A-2010/129066.
In the prior art a number of procedures different from the Knoevenagel reaction have been disclosed to obtain cyanoacrylate monomers with improved yield and purity, as well as to expand the range of monomers.
Thus, in the U.S. Pat. No. 3,654,340 it is disclosed the condensation of formaldehyde with a cyanoacetate in the presence of a mixture of an acid and a salt of a primary or secondary amine with the same acid or with a stronger acid. This procedure leads to obtaining a prepolymer, which must be thermally depolymerized to produce the monomer.
In the international patent application WO-A-94/15907 it is disclosed the esterification of 2-cyanoacryloyl chloride with an alcohol or with a diol to obtain biscyanoacrylates. Said process involves several drawbacks such as a fact of using corrosive and moisture sensitive reagents to prepare said acid chloride. Furthermore, the cyanoacrylic acid should be obtained by pyrolysis of an ester, probably obtained from the Knoevenagel reaction.
In the international patent application WO-A-2008/050313 a process for the preparation of electron deficient olefins is disclosed, wherein some specific iminium salts are used, called ionic liquids, in stoichiometric amounts, as the iminium salt is the carrier of the methylidene group of the cyanoacrylate. In said process the monomer is directly obtained by reaction of the iminium salt with a cyanoacetate, so the thermal depolymerization is avoided, but it has the disadvantage that the ammonium salt, derived from the iminium salt, remains as a residue after the distillation and it must be recovered and treated to regenerate the amine to subsequently obtain the corresponding iminium salt.
In the U.S. Pat. No. 7,718,821 a method for the preparation of electron-deficient olefins such as a cyanoacrylate is disclosed, wherein an iminium salt is used in a stoichiometric amount, since said salt provides the methylidene group of the cyanoacrylate. Such iminium salt is obtained by reaction between an aldehyde and a primary amine, and subsequent protonation with an acid. When an unprotonated iminium salt is employed, such as the iodide Eschenmoser's salt (N,N-dimethylmethylideneammonium iodide), the reaction proceeds in a low yield. In said process the monomer is also obtained directly by reaction of the protonated iminium salt with a cyanoacetate, so that the thermal depolymerization is avoided. However, the residue cannot be directly reused and the ammonium salt thus obtained must be treated to regenerate the amine and this, in turn, form the corresponding iminium salt by reaction with paraformaldehyde. This problem of the residual by-product is further aggravated when the iminium salt is used in excess relative to the cyanoacetate, as disclosed, for example, in the International Patent Application WO-A-2013/113037.
Thus, there remains a need to provide an alternative process for the preparation of 1,1-disubstituted ethylene monomers that allows the direct preparation of such monomers from simple, readily available and non-toxic starting products, and that is applicable to a wide range of monomers.